Flies are among the most visual of insects as they barnstorm through
the air and engage one another in aerial dogfights.

So what better way to study their quick maneuvering than to put
them in a virtual reality chamber and record how they bank and
roll in response to changing images.

Using just such a virtual reality chamber, Michael Dickinson and
his Berkeley colleagues have solved a long-standing puzzle about
how flies fly. For being such a visual creature, no one could
find a direct connection between the flys visual system and the
muscles that control its single set of wings.

In the April 10 issue of Science, Dickinson, an assistant professor
of integrative biology, reports that information from the eyes
feeds instead to vestigial organs called halteres  the evolutionary
remnants of a second set of wings  that act as the flys gyroscope.
The halteres relay signals to the wing muscles to alter their
stroke or angle.

This seemingly illogical system, relaying visual information through
a muscle-bound organ, nevertheless is extremely fast. House flies
can change course in response to visual images within an amazingly
short 30 milliseconds.

We knew behaviorally that a lot of flight is under the voluntary
control of vision, but weve had difficulty identifying functional
connections  wing steering muscles dont respond to changing
visual images, says Dickinson, a neuro-ethologist. But when
we looked at the steering muscles of the halteres and presented
the animal with a visual pattern, we got robust activation of
the muscles.

The discovery is important not only for what it tells us about
how flies fly and how they evolved, but also for the novel tip
it gives designers on how to stabilize small, insect-like robots
during flight.

Unlike most flying insects, flies have a single pair of wings.
The hindwings have diminished in size to millimeter long, lollipop-like
organs called halteres that beat like a normal wing during flight
but play an entirely different role. They essentially act as gyroscopes,
telling the fly how its body is rotating and sending signals to
the wing muscles to correct its orientation. They are analogous
to the human inner ear, which is critical to maintaining equilibrium.

The halteres, beating out of sync with the forewings, are the
key to the flys aerodynamic prowess.

Flies are the most accomplished fliers on the planet in terms
of aerodynamics, Dickinson says. They can do things no other
animal can, like land on ceilings or inclined surfaces. And they
are especially deft at takeoffs and landings  their skill far
exceeds that of any other insect or bird.

Dickinson has been studying how the sensory cells at the base
of the haltere detect changes in the halteres position resulting
from forces exerted during flight.

The major factor is the Coriolis force, which pushes things sideways
as they move on a rotating body. This force, which causes winds
on the spinning Earth to curl into eddies and cyclones, pushes
the beating haltere to the side when the flys body rotates. The
sensory cells, called campaniform sensilla, then send signals
to the steering neurons of the wing to alter the reflex beating
to stabilize flight or change direction.

Remove a flys halteres and it becomes unstable and quickly crashes
to the ground, he says.

The key to Dickinsons new finding was discovering a 1948 paper
in which P. F. Bonhag of Cornell University reported his dissection
of a set of tiny muscles attached to the halteres in the horse
fly. Long since forgotten, these muscles appear to be vestiges
of muscles used to steer the hindwing before it became specialized
into the sensory structures we recognize as halteres.

Though these steering muscles  11 of them in the house fly, analogous
to the 17 steering muscles attached to the flys forewing  evidently
are no longer important in generating aerodynamic forces, Dickinson
had a hunch they might be the missing connection between the visual
system and the flight muscles.

Looking at the blowfly, Calliphora vicina, he and postdoctoral
researchers Wai Pang Chan and Frederick Prete stuck glass recording
electrodes into several of the 11 minuscule steering muscles of
the haltere and measured their activity when the fly was presented
with various moving images in the virtual reality chamber.

Lo and behold, the steering muscles were strongly activated,
Dickinson says. Different muscles contracted depending upon whether
the pattern of dark lines moved up, down, across or diagonally.

He suspects that these contracting muscles tweak the halteres,
which in turn relay the effect to the wing muscles to control
flight.

Dickinson plans further studies on exactly how the steering muscles
affect the halteres.

The halteres, just one nerve cell away from the motor neurons
of the wing, are designed to react quickly  reflexively  to
yaw, pitch and roll in the fly. This allows, for example, a male
fly to rapidly change course when pursuing a female.

Moreover, it seems certain now that the halteres derived from
an earlier set of hindwings, and that flies adapted the hindwings
steering muscles to a different purpose

In many insects the forewing follows what the hindwing is doing,
and this is still going on in flies, Dickinson says. The same
basic circuitry is there in the fly, the hindwing entrains the
forewing, theyve just reused the muscles and sensors on the hindwing
in a very clever way.

The virtual reality chambers Dickinson uses in his laboratory
are cylinders lined with about 2,000 green diodes that present
black stripes moving at various angles, and at a speed between
3,000 and 4,000 frames per second. Flies are tethered to a post
in the center of the cylinder.

The high-speed images are necessary because flys eyes can see
movement 10 times faster than the human eye. In other words, while
humans see a constant image when it flickers on and off more than
30 times per second, flies do not see a continuous fused image
until the flicker rate reaches 300 times per second.

Their compound eyes, on the other hand, contain between 550 and
600 individual ommatidia (in the fruit fly) that see very little
detail.